The genomic basis of parasitism in the Strongyloides clade of nematodes

Abstract

Soil transmitted nematodes, including Strongyloides, cause one of the most prevalent Neglected Tropical Diseases. Here we compare the genomes of four Strongyloides spp., including the human pathogen S. stercoralis, and their close relatives that are facultatively parasitic (Parastrongyloides trichosuri) and free-living (Rhabditophanes sp). A significant paralogous expansion of key gene families – astacin-like and SCP/TAPS coding gene families – is associated with the evolution of parasitism in this clade. Exploiting the unique Strongyloides life cycle we compare the transcriptome of its parasitic and free-living stages and find that these same genes are upregulated in the parasitic stages, underscoring their role in nematode parasitism.

Identification, isolation, and cloning of the full-length sequence of a gene encoding trypsin inhibitor-like protein (TIL) secreted by the intestinal parasitic nematode Strongyloides ratti

“…This study agrees with the genomic study (Hunt et al, 2016) which reported that eight different gene families likely to be important in nematode parasitism in S. ratti, including that encoding trypsin inhibitorlike proteins. Fig.…”

The genomic basis of nematode parasitism

“…In this case, comparison of parasitic adult female worms with free-living adult female worms compares lifestyle, without life cycle stage being a confounding factor. Comparison of the transcriptome of these free-living and parasitic adults of S. ratti showed that the parasitic females had significantly more transcripts of astacin-metallopeptidase and SCP/TAPS coding genes-the same gene families that are comparatively enlarged in these genomes [19]. In addition to this, genes coding for transthyretin-like proteins, prolyl endopeptidases, aspartic peptidases, acetylcholinesterases, trypsin inhibitors also had comparatively greater expression in parasitic females [19].…”

“…Comparison of the transcriptome of these free-living and parasitic adults of S. ratti showed that the parasitic females had significantly more transcripts of astacin-metallopeptidase and SCP/TAPS coding genes-the same gene families that are comparatively enlarged in these genomes [19]. In addition to this, genes coding for transthyretin-like proteins, prolyl endopeptidases, aspartic peptidases, acetylcholinesterases, trypsin inhibitors also had comparatively greater expression in parasitic females [19]. Importantly, up to a third of all genes differentially expressed between these stages were hitherto unknown, hypothetical protein-coding genes, including some gene families that appear to be unique to these parasitic taxa [19], with similar phenomena reported in other species too [42].…”

“…For example, sequenced nematode genomes now range from c. 20 Mb in Pratylenchus coffeae (a pest of bananas and other crops) coding for about 7000 genes, to 370 Mb in Haemonchus contortus (a gastrointestinal parasite of sheep) coding for c. 20 000 genes [15,16]. This variation in genome size is due to changes in gene number, as well as changes in the size and number of introns, and in the extent of non-coding, intervening sequence, including repetitive sequence [16][17][18][19]. It is also interesting to note that even within the Caenorhabditis genus there are substantial differences in genome size and gene number.…”

“…Specifically, the parasitic species Strongyloides ratti and Strongyloides stercoralis have 184 and 237 astacin-like metallopeptidase coding genes, compared with 36 in the free-living relative Rhabditophanes (C. elegans has 40). For SCP/TAPS proteins, there were 89 and 113 coding genes in S. ratti and S. stercoralis, respectively, compared with 12 in Rhabditophanes (C. elegans has 36) [19].…”